1. Field of the Invention
The present invention relates to a method and apparatus for testing characteristics of a thin-film magnetic head utilized for example in a hard disk drive.
2. Description of the Related Art
A thin-film magnetic head has a write element and a read element. FIG. 6 is a sectional drawing which illustrates composition of a thin-film magnetic head. In the thin-film magnetic head 21, a write element 31 writes a signal onto a magnetic disk 50 by a magnetic field, which is generated by a current to a coil 32 in the write element 31 according to electromagnetic induction. A read element 41 reads a signal by detecting a direction of a magnetic field on the magnetic disk 50 by a magnetoresistance element 42.
FIG. 7 is a perspective view of a slider on which a thin-film magnetic head is formed. As shown in the figure, the thin-film magnetic head 21 is on an edge of the slider 2 with an electrode 22. FIG. 8A to FIG. 8C are schematic drawings of a process of manufacturing sliders. In manufacturing sliders, a lot of thin-film magnetic heads are formed on a wafer, and then the wafer is cut into bars (see FIG. 8A and FIG. 8B). Each of the bars is cut into sliders (see FIG. 8B and FIG. 8C). About eighty sliders are included in the bar. Namely, about eighty thin-film magnetic heads are in the bar. Hereinafter, each of the bars is referred to as “row-bar”.
As shown in FIG. 9A and FIG. 9B, the slider 2 is part of Head Gimbal Assembly (HGA). Namely, the slider 2 is fit to a suspension 7 (Gimbal) having a load beam 3 and a flexure 5. The load beam 3 consists of a metal plate spring. A dimple 11 projecting toward flexure 5 is on a tip side of the load beam 3. The load beam 3 and the flexure 5 are united with each other except tip sides thereof by for example welding to constitute the suspension 7. The flexure 5 includes a main body 5a and a tongue 5b which is rectangular. The tongue 5b is connected to the main body 5a at one side thereof near a tip of the main body 5a, while the other sides thereof are cut from the main body 5a. A backside of the tongue 5b is biased (pressed) by dimple 11 so that the tongue 5b is parallel to the load beam 3. The slider 2 is fixed on the tongue 5b and biased by dimple 11 through the tongue 5b so that a position thereof is appropriately maintained to write and read a signal.
In a hard disk drive (HDD), a signal is written on and read from a magnetic disk while the magnetic disk, which is rotating at high speed, and a slider of an HGA are not touching each other. A final test of the HGA should be done in a same condition as a signal is actually written on and read from a magnetic disk in. The final test is often called a dynamic performance test. Various methods for the dynamic performance test of an HGA are suggested. Generally, a result of writing (or recording) a signal on and reading (or reproducing) a signal from a medium, which may be only for a test and may not be for an actual use, by an HGA is compared with an original signal, and then characteristics of the HGA is evaluated based on an output level and lack of a bit. Hereafter, the medium, which may be only for a test and may not be for an actual use, is referred to as “false medium”.
An apparatus for testing characteristics of an HGA is disclosed in Japanese Patent Application Laid-Open No. 2002-373476.
In case an HGA does not pass the final test, expensive parts thereof like suspension and so on should be abandoned. That causes a cost increase. Therefore, it is desirable that a rate at which an inferior HGA are found in the final test is lowered as much as possible. It is thought that a thin-film magnetic head of bad quality is one of the reasons for the HGA not to pass the final test. One of the factors that the thin-film magnetic head is bad quality is an inferior write element or an inferior read element.
Concerning a read element, characteristics (static characteristics) can be obtained by applying an external magnetic field so a quasi-static-test can be done not only in a process after a row-bar is cut into sliders but also in a former process where the read element is included in a row-bar or a wafer. Therefore, there is a relatively high probability that a slider having a thin-film magnetic head whose read element is bad quality can be removed in a former process. Namely, there is a relatively low probability that a read element is bad quality is in the final test of an HGA.
On the other hand, when evaluating characteristics of a write element, a gap between a magnetic disk and a thin-film magnetic head should be controlled with precision, so it is difficult to test a write element before the final test of the HGA. Therefore, in the present state, whether a write element is good or bad has to be judged only in the final test of the HGA. This means that the probability of an inferior write element in the final test of the HGA is higher than that of an inferior read element.
Above mentioned problem is serious especially in perpendicular magnetic recording (PMR) because a yield rate in PMR is lower than that in longitudinal magnetic recording (LMR). The reason is that record density in PMR is higher (namely, energy distribution therein is narrower) than that in LMR. Further, in PMR a gap between a magnetic disk and a thin-film magnetic head should be controlled in nanometers, so it is more difficult in PMR than in LMR to test a write element before the final test of the HGA. Also, even in PMR there is a relatively low probability, as above mentioned, that a read element is bad quality in the final test of an HGA because the principle of reading a signal in PMR is same as that in LMR.